An Introduction to Nature-study

by E. Stenhouse

But the succulent plants which the rabbit eats are not suddenly transformed into animal muscle and bone, and so forth, when they are swallowed. They have first of all to undergo a process which is called digestion. This takes place in a tube—the digestive canal—which runs from end to end of the body. The digestive canal of the rabbit is coiled in a somewhat intricate manner. That of the frog is, however, much simpler and more typical of vertebrates ([p. 220]) generally, and will serve equally well in so elementary a consideration of digestion as the present.[11]

The frog’s digestive canal.—The hinder end of the frog’s mouth opens ([Fig. 163]) into the gullet (gul.), a short wide tube which leads to a capacious bag called the stomach (st.). The termination of this is continuous with a coiled narrow tube called the small intestine (s. in.), which passes suddenly into a much wider large intestine or rectum. The rectum opens to the exterior, at the hinder end of the body, by an aperture (an.) known as the anus. In addition to the digestive tube proper, two large digestive glands, the liver and the pancreas, should be carefully noticed. The liver (lr.) is a large, dark red organ, consisting of two lobes which lie at the sides of the stomach. It makes a digestive fluid called bile. The pancreas (pn.) is an elongated body which lies in the loop between the stomach and the first portion—called the duodenum (du.)—of the small intestine. It makes a digestive fluid called the pancreatic juice. In the frog both the bile and the pancreatic juice are discharged into the duodenum by one tube or duct (b.d.). Small digestive glands also occur in the inner wall of the stomach; these discharge a fluid called gastric juice into the cavity of the stomach.

Fig. 163.—The Frog, dissected from the left side. gul, gullet; st, stomach; du, duodenum; s. int, small intestine; an. anus; lr, liver; pn, pancreas; b.d, bile duct; bl, urinary bladder; c.art, l.au, s.v, v, parts of the heart; cn. 3, “body” of 3rd vertebra; eus t. Eustachian tube; gl, glottis, leading to l.lng, left lung, and r.lng, right lung; n.a. 1, arch of 1st vertebra; p.na, internal opening of nostril; sp.cd, spinal cord; spl. spleen; tng, tongue; kd, kidney; ur, ureter; vo.t, vomerine teeth.

The rabbit’s digestive canal—In its main features the digestive canal of the rabbit resembles that of the frog; here also the mouth opens into a long tube consisting of gullet, a bag-like stomach, a small intestine, and a large intestine. There are also a liver and a pancreas which discharge their fluids—by separate ducts, however,—into the first part of the small intestine; and the stomach is supplied with gastric juice by small glands in its inner wall. There are, however, certain differences besides those of size in the digestive tubes of the two animals. In the first place, a fluid called saliva is poured into the rabbit’s mouth by the ducts of salivary glands which occur near the mouth. Secondly, the coils of the small intestine are very much more complicated than in the frog. And lastly, at the junction of small and large intestines there is given off, in the rabbit and in herbivorous mammals generally (when these have simple stomachs,—[p. 261]), a great, spirally-constricted tube which ends blindly in a finger-like process.

How the rabbit digests its food.—A rabbit needs food to repair the constant waste of substance which the activities of its life entail; and the same is true of every other living thing, be it plant or animal. Now, every part of a rabbit’s body is irrigated and drained by the finest branches of a system of pipes through which blood is always flowing; and it is in this blood-stream that the food is conveyed to the muscles and other organs which are to be repaired. The food finds its way into the blood when that fluid is flowing through the small vessels which lie in the thickness of the wall of the digestive tube. Before food can gain access to the blood it must be in a condition in which it is capable of diffusing through the thin membrane which separates them. Digestion is the process which renders food soluble and diffusible, and hence capable of passing into the blood. The food of animals is of several different kinds. A few of these are soluble and diffusible at the time they are eaten, but most of them are neither, and therefore require treating according to their nature. This is why so many different fluids are poured into the rabbit’s digestive canal. Saliva, gastric juice, bile, and pancreatic juice are each capable of acting upon certain constituents of the food and rendering them soluble and diffusible.

It would be beyond the scope of this book to consider the work of these fluids in detail, but the action of saliva is not only fairly typical, but it can easily be imitated outside the body. Starch is a very common constituent of vegetable foods, and its presence or absence is readily determined by the blue colour which it gives with a solution of iodine. Starch is quite insoluble in cold water, but when treated with hot water it swells up and, to a great extent, dissolves. But a mere solution of starch cannot get into the blood, for it is incapable (Expt. 45, 1) of passing through a thin membrane. On the other hand, if starch is mixed with saliva, and the mixture is kept at the temperature of the body, it is found in a short time that the starch has been changed into sugar, which is not only soluble, but readily diffuses through a thin membrane. In other words, starch as such is useless to the rabbit as food; only after it has been digested by conversion into sugar can it be used by the body.

Something very similar often takes place in plants. It was seen ([p. 33]) that the cotyledons of a pea become sweet during germination. Starch is a convenient form of food for storing in the cotyledons of a pea, the endosperm of the maize seed, the short stem of the crocus-corm, and so on; but before the plant can use it as food the starch must be made soluble and diffusible by being changed into sugar. In plants the change is brought about, not by saliva, but by a substance known as diastase.

The pancreatic juice continues the change of starch into sugar which is commenced by the saliva, but it also digests other food-stuffs as well. Similarly, gastric juice and bile are each concerned with the digestion of certain foods. The result of the action—separate and combined—of the digestive fluids is that, when a rabbit eats no more food than it requires, all the useful parts of the food are absorbed into the blood, and then distributed to the tissues by the blood as it flows through them.